Ophthalmic or eyeglass lens blanks are subjected to a variety of grinding and polishing steps in order to create a finished eyeglass lens. During these processing steps the lens is shaped to the proper curvature, thickness and prism. Prior to processing, the lens is circular, of a particular thickness and opposite sides are parallel. To obtain the proper optical effects in order to correct a patient's vision, the opposite surfaces are shaped so that one surface is concave and the other surface is convex. In addition, a prism correction is also frequently needed. The prism correction is obtained by altering the parallel nature of the lens blank and grinding one surface to create an angle between the other surface. Thus, a lens blank that includes a prism correction or component is thicker at one end of its diameter then at the opposite end.
The problem of obtaining the precise alignment between the opposite surfaces has been attacked in a variety of ways. In one such solution the lens blank is attached to the lens blocking body in the conventional manner using pitch or another acceptable adhesive. Then, the back side of the lens blocking body is attached to the grinding machine chuck, also in the conventional manner, however, a shim is inserted therebetween to present the lens at the desired angle to the grinding wheel.
Another apparatus for introducing prism into lenses is disclosed in Cradlock U.S. Pat. No. 3,273,204. A mold for the manufacture of lenses is disclosed and includes two opposing housings, each having a bore that receives a lens molding die. One of the axes of the bore is offset in relation to the common axis and one of the dies is retractable within its housing to obtain the desired amount of prism.
The foregoing devices are generally effective for fabricating lenses having low (generally considered less than 4 diopters) corrections, however, lenses having a correction greater than 4 diopters are difficult to manufacture and require great skill to obtain satisfactory results.
Another apparatus for generating a lens having a prism is disclosed in Knight U.S. Pat. No. 3,866,667. Specifically, the patent teaches a blocking device in which a pair of wedge-shaped prism rings may be adjusted relative to each other and relative to the bore of the device for incorporating into the lens block a predetermined amount of prism at a predetermined meridional orientation. Graduations, measured in diopters are marked on the rings. In this system, the entire blocking body is molded and is generally effective in inducing large amounts of prism. However, the Knight device was designed for use with glass lens blanks and when used with plastic lenses, warpage occurs as the device retains too much heat.
It would, therefore, be advantageous to industry and to optical laboratories in particular, to be able to reliably produce ophthalmic spectacle lenses having a high prism correction.